1. Field of the Invention
The present invention relates to a storage cell or secondary cell in electrochemical cells, and more particularly, to a polymer secondary cell and a lithium ion secondary cell.
2. Description of the Related Art
There are known a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery and a lithium ion battery as secondary batteries. In recent years, the lithium ion storage cell has attracted attention because of the high energy density and the high potential thereof for allowing a lightweight storage battery.
For example, the lithium ion storage cell comprises; a positive electrode having a positive electrode layer of LiCoO.sub.2 or the like formed on a positive electrode collector; a negative electrode having a negative electrode layer of graphite or the like formed on a negative electrode collector; and a separator. The lithium ion storage cell is constructed in a such a manner that the positive and negative electrodes are wrapped while separated by the separator in a container filled with an organic electrolyte containing LiPF.sub.6 or the like and sealed. In addition, the separator for insulating the positive and negative electrodes consists of a porous insulator so as not to inhibit ion transmission within the electrolyte. For example, during charging, Li cations move from the positive electrode layer to the negative electrode layer through the separator, while during discharging, Li cations move from the negative electrode layer to the positive electrode layer through the separator.
Another lithium ion storage cell has also been developed in which a polymer positive electrode is used instead of LiCoO.sub.2 of the positive electrode for reduced weight and greater safety. In this lithium ion storage cell, the use of polyaniline as the positive electrode active substance has attracted attention in recent years due to its excellent environmental safety, large charging and discharging capacity and excellent durability.
Electrochemical and chemical oxidation processes are known as methods for producing the polyaniline used for the positive electrode of a non-aqueous secondary cell such as a lithium ion storage cell.
In the case of producing polyaniline using an electrochemical process, the polymer is obtained in the form of a film on the positive electrode, but this is industrially disadvantageous since it requires a large amount of electrical energy. In addition, since the resulting porous film has a low level of strength and easily comes off the electrode, various types of post-processing are difficult. It is also not easy to produce the film having a large surface areas for the secondary cell.
On the other hand, in the case of using a chemical oxidation process, since the polyaniline is normally obtained in the form of a powder, the polyaniline powder must be processed by means such as a press forming so as to use it as the positive electrode for a secondary cell. Moreover, the resulting press formed product is brittle and easily broken. Thus, it is not easy to form the polyaniline into a thin film. Moreover, in the case of using the polyaniline powder in the form of an organic solution and obtaining a film from that solution, the density of the film is high, and particularly when it has a large thickness, the charging and discharging characteristics of the lithium ion cell tend to be extremely inferior.
In a lithium ion storage cell in which graphite is used as the negative electrode, in the case of using a propylene carbonate-based electrolyte, the propylene carbonate decomposes as a result of charging and discharging, resulting in extremely poor cycle characteristics. In the case of using an ethylene carbonate-based electrolyte, cycle characteristics are improved. However, since ethylene carbonate has a high melting point of 36.4.degree. C., discharge capacity at a low temperature is extremely small.
Moreover, in the case of using the conductive polymer e.g., polyaniline, for the positive electrode, the lithium ion storage cell does not operate at a low temperature simply by raising the ion conductivity of the electrolyte at a low temperature by increasing the ratio of low viscosity solvent of the electrolyte.